…shboard; #1452's #ci routing adopted by pointing the dashboard at #ci)

The full per-PR breakdown was huge in-channel on busy days. The
channel message is now a one-line summary with counts; the breakdown
lives in a single threaded reply, both updated in place (parent update
passes blocks: [] since chat.update retains existing blocks otherwise).

Channel moves from #building to #ci, adopting #1452's routing decision
for merge announcements (merged from main; the old workflow it edited
stays deleted).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Replaces the `merge-to-main-slack` workflow (one Slack message per
merged PR — noisy on busy days) with a workflow that maintains **at most
one message per day** in `#ci`: a one-line PR dashboard summary, with
the full per-PR breakdown in a single threaded reply. Both are created
on the first PR event of the day and updated in place after that.

Channel message:

> **PR dashboard 10th June** — 51 merged · 9 closed without merging · 4
opened · 2 older still open (details in thread)

Threaded reply (rendered from real data):

> **Merged:**
> • [#1410 Fix 5-min logout, deploy-time JWKS, and stream append
skeleton flash](#1410) by jonas
(ad6da76)
> • [#1407 itx: contexts, capabilities, and the one true
handle](#1407) by jonas (f256768)
> …
> **Closed without merging:**
> • [#1440 Migrate captun to published npm
0.0.3](#1440) by misha
> …
> **Opened:**
> • [#1448 Replace per-merge Slack messages with a daily PR
dashboard](#1448) by misha
(draft)
> …
> Old: [#1349](#1349),
[#1355](#1355)

How it works:

- Content is refetched from the GitHub search API on every run (merged /
closed-unmerged / opened-and-still-open today, plus older open PRs), so
the message is self-healing — no incremental state to corrupt.
- The day's message timestamps live in a repo Actions variable
(`SLACK_PR_DASHBOARD_STATE`, `{date, channel, ts, details_ts}`), written
with the same `ITERATE_BOT_GITHUB_TOKEN` the nag workflow uses. No new
Slack scopes needed: `chat.update` uses the `chat:write` the bot already
exercises.
- Targets `#ci`, adopting #1452's decision to move merge announcements
out of `#building` (that PR edited the workflow this one deletes; the
conflict is resolved here by keeping the deletion).
- The threaded details go out as chunked mrkdwn section blocks rather
than one `text` param: on busy days a single text field hits
`chat.update`'s `msg_too_long` (`postMessage` truncates, `update`
rejects — found by e2e-testing against today's ~50 merges).
- Plain-text author names (no @-mentions) since the messages update many
times a day.
- Testable two ways: pushing any `*pr-dashboard*` branch runs it for
real against `#misha-test` with a separate state variable (create,
update-in-place, and threading paths all verified this way — e.g. runs
[27280068182](https://github.com/iterate/iterate/actions/runs/27280068182),
[27288814028](https://github.com/iterate/iterate/actions/runs/27288814028)),
and `node cli.ts github-script
pr-dashboard.update_dashboard.update_pr_dashboard --github-token ...`
does a local dry run that prints both messages.

Task file: `tasks/slack-daily-pr-dashboard.md`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>